Methods of fabricating dual threshold voltage devices

ABSTRACT

An annular device is provided. The annular device includes a first transistor including a first input terminal and a second transistor including a second input terminal. The first input terminal and the second input terminal extend radially outward from the annular device, and wherein the first input terminal is aligned with the second input terminal.

PRIORITY AND RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/865,140, entitled “Methods of Fabricating Dual Threshold VoltageDevices,” filed Jan. 8, 2018,” which is incorporated by reference hereinin its entirety.

This application is related to U.S. patent application Ser. No.15/865,125, entitled “Adjustable Current Selectors,” filed Jan. 8,2018,” U.S. patent application Ser. No. 15/865,138, entitled “DualThreshold Voltage Devices with Stacked Gates,” filed Jan. 8, 2018,” U.S.patent application Ser. No. 15/865,135, entitled “Dual Threshold VoltageDevices,” filed Jan. 8, 2018,” U.S. patent application Ser. No.15/865,132, entitled “Methods of Fabricating Dual Threshold VoltageDevices with Stacked Gates,” filed Jan. 8, 2018,” U.S. patentapplication Ser. No. 15/865,123, entitled “Methods of FabricatingContacts for Cylindrical Devices,” filed Jan. 8, 2018,” U.S. patentapplication Ser. No. 15/865,144, entitled “Dual Gate Memory Devices,”filed Jan. 8, 2018,” each of which is incorporated by reference hereinin its entirety.

TECHNICAL FIELD

This relates generally to the field of memory applications and voltagedevices, including but not limited to dual threshold voltage devices.

BACKGROUND

The field of memory applications is becoming more challenging as theperformance requirements for memory-based devices increase. Because ofmany useful properties of dual threshold voltage devices (e.g.,adjustability, density, and drivability), memory systems comprising dualthreshold voltage devices have superior performance over conventionalmemory systems.

SUMMARY

There is a need for systems and/or devices with more efficient,accurate, and effective methods for fabricating and/or operating memorysystems. Such systems, devices, and methods optionally complement orreplace conventional systems, devices, and methods for fabricatingand/or operating memory systems.

The present disclosure describes a dual threshold voltage device, alsosometimes called a dual gate device. For example, a device is provided,the device comprising a core, a plurality of layers that surround thecore in succession, including a first layer, a second layer, a thirdlayer, and a fourth layer. The core, the first layer, and the secondlayer correspond to a first transistor. In some embodiments, the core isthe gate of the first transistor, the first layer is the gate dielectricof the first transistor, and the second layer serves as a channel of thefirst transistor as well as a channel of the second transistor. Thesecond layer, the third layer, and the fourth layer correspond to asecond transistor. In some embodiments, the third layer is a gatedielectric for the second transistor and the fourth layer is a gate ofthe second transistor. It is important to note that the second layer isa common channel for the first and second transistors. The devicefurther comprises a first input terminal coupled to the core, the firstinput terminal being configured to receive a first voltage and a secondinput terminal coupled to the fourth layer, the second input terminalbeing configured to receive a second voltage. The device furthercomprises a common source terminal coupled to the core and the fourthlayer. In other words, the device further comprises a common sourceterminal for both transistors which is a lower portion of the commoncylindrical channel. The upper portion of the cylindrical channel servesas a common drain for the first and second transistors. In someimplementations, a memory device, such as an MTJ is coupled to thechannel drain of the device (e.g., the dual Vt transistors). Thus, insome implementations, a device for easily programing an MTJ, the devicehaving two threshold voltages, is provided. An exemplary device isprovided in FIG. 5.

In one aspect, some implementations include a method of fabricating anannular device (e.g., an annular dual threshold voltage device). Themethod comprises providing a cylindrical device having a conductive corecorresponding to a first transistor and a plurality of annular layerssurrounding the core, including a first dielectric layer, a secondlayer, a third dielectric layer, and a fourth conductive layercorresponding to a second transistor and creating a silicide source,wherein the silicide source is coupled to the conductive corecorresponding to the first transistor and the fourth conductive layercorresponding to the second transistor. The method of creating thesilicide source includes coating a first oxide substrate with an organicpolymerizing layer (OPL) to create an OPL plane, wherein the OPL planesurrounds a horizontal cross section of the cylindrical device anddepositing low-temperature silicon oxide (LTO) on the OPL to create anLTO layer, wherein the LTO layer surrounds a horizontal cross section ofthe cylindrical device. The method of creating the silicide sourcefurther includes coating the LTO layer with a bottom antireflectivecoating (BARC) layer and a photoresistive (PR) layer, applying a layerof silicide to create the silicide source and creating a mask with thePR layer, wherein the mask covers the second layer of the cylindricaldevice. The method further includes dry etching the BARC layer, the LTOlayer and the OPL until the oxide substrate is exposed and then etchinghorizontally the LTO layer and oxide to expose a layer of silicon on ahorizontal cross section of the cylindrical device to create an annulararea. The method further includes Ion Implanting a source dopant andperforming a first rapid thermal annealing (RTA) on the annular area toreduce its electrical resistance and then depositing a siliciding metalon the annular area to create the silicide source and performing asecond RTA to reduce its electrical resistance.

In another aspect, some implementations include a method for fabricatingan annular device, including an annular polycide layer. The methodcomprises forming a cylindrical device having a conductive corecorresponding to a first transistor and a plurality of annular layerssurrounding the core, including a first dielectric layer, a secondlayer, a third dielectric layer, and a fourth conductive layercorresponding to a second transistor. The method further comprisescoating a spin-on glass (SOG) layer on a first plane and the cylindricaldevice to create a sloped ring around the bottom of the cylindricaldevice, wherein the cylindrical device is vertically disposed in thefirst plane and the SOG surrounds the cylindrical device. The methodfurther comprises etching the SOG layer from around and inside thecylindrical device to a desired depth and depositing dielectricmaterials to form the third dielectric layer. The method furtherincludes depositing doped chemically vaporized polysilicon on the firstplane, a horizontal cross section of the cylindrical device, a top ofthe cylindrical device, and the sloped ring around the cylindricaldevice (e.g., using chemically vaporized deposition). The method furtherincludes etching, using a reactive-ion etch (RIE), the polysilicon onthe top of the cylindrical device and on the sloped ring and depositinga siliciding metal and performing rapid thermal anneal (RTA) to createthe fourth conductive layer of reduced electrical resistivity.

In another aspect, some implementations include a method for fabricatingan annular device, including creating a contact between a required nodeof the device to other metal connections. Some aspects of the methodinclude providing a cylindrical device having a conductive corecorresponding to a first transistor and a plurality of annular layerssurrounding the core, including a first dielectric layer, a secondlayer, a third dielectric layer, and a fourth conductive layercorresponding to a second transistor, wherein the fourth conductivelayer is an outermost layer of the cylindrical device. Some methodsinclude converting the fourth conductive layer of the CVD polysilicon toa low resistivity silicide a gate of the second transistor. This mayinclude depositing a first low-temperature oxide (LTO) layer to create afirst LTO plane and spin coating a spin-on glass (SOG) layer in thefirst LTO plane, wherein the cylindrical device is vertically disposedin the first LTO plane. The method for making a contact to theconductive layer for the gate of the second transistor includes etchingthe SOG layer to a first thickness and spin coating an organic compoundto a first height to create an organic planarizing layer (OPL), whereinthe OPL surrounds a horizontal cross section of the cylindrical device.The method further includes depositing a second LTO layer onto the OPL,wherein the second LTO layer surrounds a horizontal cross section of thecylindrical device, and coating the second LTO layer with a bottomanti-reflective coating (BARC) layer. The method further includescoating the BARC layer with a photoresist layer, wherein the BARC layerand the photoresist layer surround a horizontal cross section of thecylindrical device and creating a mask with the photoresist layer. Themethod further includes dry etching the BARC layer and the second LTOlayer using a fluorine-based chemistry through the BARC layer and thesecond LTO layer into the OPL, wherein the dry etching areas is definedby the mask layout of the photoresist pattern layer. The method furtherincludes etching the OPL in oxygen plasma until the SOG layer is exposedto create a trench and depositing tantalum nitride in the trench tocreate a contact from the silicided polysilicon of the gate to the metallines needed for the memory device or connection to the logic devices.

In another aspect, some implementations include a method for fabricatingan annular device, including creating a contact between a core of adevice and any other metal line for connection to a given core. Themethod comprises providing a cylindrical device having a conductive corecorresponding to a first transistor and a plurality of annular layerssurrounding the core, including a first dielectric layer, a secondlayer, a third dielectric layer, and a fourth conductive layercorresponding to a second transistor. The method further comprisescoupling (or providing access to couple) the conductive core of thecylindrical device to other metal nodes. This includes depositing, bychemical vapor deposition (CVD), a first silicon nitride (SiN) layer tocreate a first SiN plane. The method further includes depositing aspin-on glass (SOG) layer and a low temperature oxide (LTO) layer andperforming a metal gate contact mask. The method further includesetching, using an ion beam trench etch, to create a trench through theplurality of annular layers surrounding the core and depositing a secondSiN layer in the trench and on sides of the trench. The method furtherincludes removing the deposited first SiN layer from the core by a firstchemical mechanical polishing (CMP) and depositing a metal gate contactand metal lines to create a contact from the conductive core to outermetal lines.

Thus, devices and systems are provided with methods for fabricating andoperating dual threshold (e.g., dual gate) devices, thereby increasingthe effectiveness, efficiency, and user satisfaction with such systemsand devices.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the various described implementations,reference should be made to the Description of Implementations below, inconjunction with the following drawings in which like reference numeralsrefer to corresponding parts throughout the figures.

FIG. 1A shows a schematic diagram of a representative magnetic tunneljunction (MTJ) structure in accordance with some implementations.

FIG. 1B shows representative energy barriers of the reference andstorage layers of the MTJ of FIG. 1A in accordance with someimplementations.

FIGS. 2A-2B illustrate magnetization orientations in a representativeperpendicular magnetic tunnel junction (pMTJ) structure in accordancewith some implementations.

FIGS. 3A-3D illustrate representative processes for switching the pMTJof FIGS. 2A-2B between the parallel and anti-parallel configurations inaccordance with some implementations.

FIG. 4 is a schematic diagram of a representative spin transfer torque(STT) MRAM device in accordance with some implementations.

FIG. 5 illustrates a dual threshold voltage device in accordance withsome implementations.

FIG. 6 illustrates a dual threshold voltage device with aligned gatehandles in accordance with some implementations.

FIG. 7 is a schematic diagram of a representative circuit in accordancewith some implementations.

FIG. 8 is a schematic of a dual threshold voltage device and MTJstructure in accordance with some implementations.

FIGS. 9-15 illustrate a process for forming a silicide source inaccordance with some implementations. FIGS. 9A-9C illustrate a processof forming a channel in accordance with some implementations. FIGS.10A-10B illustrate a doping process in accordance with someimplementations. FIGS. 11A-11C illustrate a process of forming annularlayers in accordance with some implementations. FIGS. 12A-12C illustratean etching process in accordance with some implementations. FIGS.13A-13B illustrate an annular device in accordance with someimplementations. FIG. 14 illustrates a schematic view of both anN-device and a P-device in accordance with some implementations. FIGS.15A-15C illustrate a siliciding process in accordance with someimplementations.

FIGS. 16-18 illustrate processes for forming an annular transistor inaccordance with some implementations. FIGS. 16A-16C illustrate a processof forming a gate structure on an annular device in accordance with someimplementations. FIGS. 17A-17B illustrate a process of depositingpolysilicon on an annular device in accordance with someimplementations. FIG. 18 illustrates a post-etching state of the annulardevice in accordance with some implementations.

FIGS. 19A-19B illustrate a process for creating a gate handle for atransistor in accordance with some implementations.

FIGS. 20A-20D illustrate a device having a source and an annulartransistor in accordance with some implementations.

FIGS. 20E-20G illustrate a representative process of planarizing arepresentative device in accordance with some implementations.

FIGS. 20H-20K illustrate a representative process for adjusting heightsof a representative device in accordance with some implementations.

FIGS. 21-23 illustrate a representative process for creating a contactfor a transistor located in the center of an annular device inaccordance with some implementations. FIGS. 21A-21G illustrate aplanarization process for an annular device in accordance with someimplementations. FIGS. 22A-22C illustrate a process for etching anannular device in accordance with some implementations. FIGS. 23A-23Cillustrate a process for contacting a layer of an annular device inaccordance with some implementations.

FIG. 24 illustrates an array of dual threshold voltage devices inaccordance with some implementations.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION

Reference will now be made in detail to implementations, examples ofwhich are illustrated in the accompanying drawings. In the followingdetailed description, numerous specific details are set forth in orderto provide a thorough understanding of the various describedimplementations. However, it will be apparent to one of ordinary skillin the art that the various described implementations may be practicedwithout these specific details. In other instances, well-known methods,procedures, components, circuits, and networks have not been describedin detail so as not to unnecessarily obscure aspects of theimplementations.

A device, and a method of fabricating the device, having dual thresholdvoltages is provided. The device has a core, a plurality of layers thatsurround the core in succession, including a first layer, a secondlayer, a third layer, and a fourth layer. The core, the first layer, andthe second layer correspond to a first transistor. The second layer, thethird layer, and the fourth layer correspond to a second transistor. Thesecond layer is a common channel for the first and second transistors.The device further comprises a first input terminal coupled to the core,the first input terminal being configured to receive a first voltage anda second input terminal coupled to the fourth layer, the second inputterminal being configured to receive a second voltage. The devicefurther comprises a common source terminal coupled to the core and thefourth layer (but not physically or electrically shorting). In someimplementations, a memory device, such as an MTJ is coupled to the drainside of the channel of the device. Thus, in some implementations, adevice and a method of fabrication of the device that can be used toeasily and efficiently program an MTJ is provided.

In some implementations, dual threshold voltage, dual transistor CMOSdevices are provided. In some implementations, cylindrical pillars areetched out of silicon for CMOS devices (e.g., <100> silicon). In someimplementations, the channels are annular vertical cylinders etched intosilicon. In some implementations, the source comprises bottom silicidedareas of the annular cylinder of silicon and the drain is the top partof the channel. In some implementations, the innermost cylinder makesthe first transistor (e.g., Gate 1) electrode with a first thresholdvoltage (e.g., Vt1). In some implementations, the outer wrap around thepoly cylinder makes the second transistor (e.g., Gate 2) with a secondthreshold voltage (e.g., Vt2). In some implementations, the firstthreshold voltage and the second threshold voltage of the N and/or Ptype devices (e.g., transistors) can be individually tailored by properchoices of: the dopant of the cylindrical pillar, the thickness of thegates' dielectrics, the dielectric constant of their gates' materials,and the/or the work functions of the gate electrodes. In someimplementations, the thicker the cylindrical pillar means the greaterthe drive currents will be. In some implementations, the combined drivecurrent is an order of magnitude more than a surface device with a givenphotolithographic step (F, minimum feature size). In someimplementations, the shallower the pillar, the faster will be the speedof the vertical channel transistors. In some implementations, having adual threshold voltage device saves real estate (e.g., space) in thelayout in analog CMOS circuits, digital, and/or memory.

Magnetic Memory Devices

Magnetoresistive random access memory (MRAM) is a non-volatile memorytechnology that stores data through magnetic storage elements. MRAMdevices store information by changing the orientation of themagnetization of a storage layer. For example, based on whether thestorage layer is in a parallel or anti-parallel alignment relative to areference layer, either a “1” or a “0” can be stored in each MRAM cell.

The present disclosure describes various implementations of MRAM systemsand devices. As discussed in greater detail below, MRAM stores datathrough magnetic storage elements. These elements typically include twoferromagnetic films or layers that can hold a magnetic field and areseparated by a non-magnetic material. In general, one of the layers hasits magnetization pinned (e.g., a “reference layer”), meaning that thislayer requires a large magnetic field or spin-polarized current tochange the orientation of its magnetization. The second layer istypically referred to as the storage, or free, layer and itsmagnetization direction can be changed by a smaller magnetic field orspin-polarized current relative to the reference layer.

Due to the spin-polarized electron tunneling effect, the electricalresistance of the cell changes due to the orientation of themagnetization of the two layers. A memory cell's resistance will bedifferent for the parallel and anti-parallel states and thus the cell'sresistance can be used to distinguish between a “1” and a “0”. Oneimportant feature of MRAM devices is that they are non-volatile memorydevices, since they maintain the information even when the power is off.In particular, the layers can be sub-micron in lateral size and themagnetization direction can still be stable over time and with respectto thermal fluctuations.

FIG. 1A is schematic diagram of a magnetic tunnel junction (MTJ)structure 100 (e.g., for use in an MRAM device) in accordance with someimplementations. In accordance with some implementations, the MTJstructure 100 is composed of a first ferromagnetic layer (referencelayer 102), a second ferromagnetic layer (storage layer 106), and anon-magnetic layer (spacer layer 104). The reference layer 102 is alsosometimes referred to as a pinned or fixed layer. The storage layer 106is also sometimes referred to as a free layer. The spacer layer 104 isalso sometimes referred to as a barrier layer (or a non-magnetic spacerlayer). In some implementations, the spacer layer 104 comprises anelectrically-insulating material such as magnesium oxide (MgO) orMgAl₂O₄.

In some implementations, the reference layer 102 and the storage layer106 are composed of the same ferromagnetic material. In someimplementations, the reference layer 102 and the storage layer 106 arecomposed of different ferromagnetic materials. In some implementations,the reference layer 102 is composed of a ferromagnetic material that hasa higher coercivity and/or thermal stability than the storage layer 106.In some implementations, the reference layer 102 and the storage layer106 are composed of different ferromagnetic materials with the same orsimilar thicknesses (e.g., within 10%, 5%, or 1% of one another). Insome implementations, the thickness of the reference layer 102 isdifferent from that of the storage layer 106 (e.g., the reference layer102 is thicker than the storage layer 106). In some implementations, thethickness of the spacer layer 104 is on the order of a few atomiclayers. In some implementations, the thickness of the spacer layer 104is on the order of a few nanometers (nm). In some implementations,thicknesses of the reference layer 102, the spacer layer 104, and thestorage layer 106 are uniform. In some implementations, thicknesses ofthe reference layer 102, the spacer layer 104, and the storage layer 106are not uniform (e.g., a first portion of the spacer layer 104 isthinner relative to a second portion of the spacer layer 104).

In some implementations, the reference layer 102 and/or the storagelayer 106 is composed of two or more ferromagnetic layers are separatedfrom one another with spacer layers. In some implementations, each ofthese ferromagnetic layers is composed of identical, or varying,thickness(es) and/or material(s). In some implementations, the spacerlayers are composed of identical, or varying, thickness(es) and/ormaterial(s) with respect to one another.

Magnetic anisotropy refers to the directional dependence of a material'smagnetic properties. The magnetic moment of magnetically anisotropicmaterials will tend to align with an “easy axis,” which is theenergetically favorable direction of spontaneous magnetization. In someimplementations and instances, the two opposite directions along an easyaxis are equivalent, and the direction of magnetization can be alongeither of them (and in some cases, about them). For example, inaccordance with some implementations, FIG. 1B shows low energy states114 and 116 corresponding to opposite directions along an easy axis(additional examples are shown in FIGS. 10A-10B with reference to acylindrical three-dimensional MTJ structure).

In some implementations, the MTJ structure 100 is an in-plane MTJ. Inthis instance, the magnetic moments of the reference layer 102 and thestorage layer 106, and correspondingly their magnetization direction,are oriented in the plane of the ferromagnetic films of the referencelayer 102 and the storage layer 106.

In some implementations, the MTJ structure 100 is a perpendicular (orout-of-plane) MTJ. In this instance, the magnetic moments of thereference layer 102 and the storage layer 106, and correspondingly theirmagnetization direction, are oriented perpendicular and out-of-plane tothe ferromagnetic films of the reference layer 102 and the storage layer106.

In some implementations, the MTJ structure 100 has preferred directionsof magnetization at arbitrary angles with respect to the magnetic filmsof the reference layer 102 and the storage layer 106.

In accordance with some implementations, an MRAM device provides atleast two states such that they can be assigned to digital signals “0”and “1,” respectively. One storage principle of an MRAM is based on theenergy barrier required to switch the magnetization of a single-domainmagnet (e.g., switch the magnetization of the storage layer 106) fromone direction to the other.

FIG. 1B shows representative energy barriers of the reference layer 102and the storage layer 106 of the MTJ 100 in accordance with someimplementations. In accordance with some implementations, the energybarrier refers the amount of energy the magnetic material must overcomein order to switch from one magnetization direction to its opposite(e.g., from the state 114 to the state 116). In an MRAM device, themagnetization direction of the reference layer 102 is generallyconsidered fixed, while the magnetization direction of the storage layer106 is varied to store the “0” and “1” states. Accordingly, thereference layer 102 is composed of materials such that an energy barrier112 (E_(B, ref)) of the reference layer 102 is larger than the energybarrier 118 (E_(B, stor)) of the storage layer 106. In particular, FIG.1B shows low energy states 114 and 116 for the reference layer 102separated by the energy barrier 112, and shows low energy states 120 and122 for the storage layer 106 separated by the energy barrier 118. Insome implementations, the storage layer 106 is designed with materialsthat have a magnetic anisotropy that is high enough to store themagnetization over certain time duration (for e.g., 1 week, 1 month, 1year, or 10 years).

For an MRAM device with the MTJ structure 100, the resistance states ofthe MRAM devices are different when the magnetization directions of thereference layer 102 and the storage layer 106 are aligned in a parallel(low resistance state) configuration or in an anti-parallel (highresistance state) configuration, as will be discussed with respect toFIGS. 2A and 2B.

FIGS. 2A-2B illustrate magnetization orientations in a perpendicularmagnetic tunnel junction (pMTJ) structure 200 in accordance with someimplementations. In some implementations, the pMTJ structure 200 is thesame as the MTJ structure 100 presented in FIG. 1A, comprising: thereference layer 102, the spacer layer 104, and the storage layer 106. Insome implementations, the pMTJ structure 200 forms part of a MRAMdevice.

For the pMTJ structure 200 illustrated in FIGS. 2A and 2B, the fixedmagnetization direction 202 for the reference layer 102 is chosen to bein an upward direction and is represented by an up arrow. In someimplementations (not shown), the fixed magnetization direction of thereference layer 102 in the pMTJ structure 200 is in a downwarddirection.

FIG. 2A illustrates the magnetization directions of the storage andreference layers in a parallel configuration. In the parallelconfiguration, the magnetization direction 206 of the storage layer 106is the same as the magnetization direction 202 of the reference layer102. In this example, the magnetization direction 202 of the referencelayer 102 and the magnetization direction 206 of the storage layer 106are both in the upward direction. The magnetization direction of thestorage layer 106 relative to the fixed layer 102 changes the electricalresistance of the pMTJ structure 200. In accordance with someimplementations, the electrical resistance of the pMTJ structure 200 islow when the magnetization direction of the storage layer 106 is thesame as the magnetization direction 202 of the reference layer 102.Accordingly, the parallel configuration is also sometimes referred to asa “low (electrical) resistance” state.

FIG. 2B illustrates the magnetization directions of the storage andreference layers in an anti-parallel configuration. In the anti-parallelconfiguration, the magnetization direction 216 of the storage layer 106is opposite to the “fixed” magnetization direction 202 of the referencelayer 102. In accordance with some implementations, the electricalresistance of the pMTJ structure 200 is high when the magnetizationdirection 216 of the storage layer 106 is the opposite of themagnetization direction 202 of the reference layer 102. Accordingly, theanti-parallel configuration is sometimes also referred to as a “high(electrical) resistance” state.

Thus, by changing the magnetization direction of the storage layer 106relative to that of the reference layer 102, the resistance states ofthe pMTJ structure 200 can be varied between low resistance to highresistance, enabling digital signals corresponding to bits of “0” and“1” to be stored and read. Conventionally, the parallel configuration(low resistance state) corresponds to a bit “0,” whereas theanti-parallel configuration (high resistance state) corresponds to a bit“1”.

Although FIGS. 2A-2B show parallel and anti-parallel configurations withthe pMTJ structure 200, in some implementations, an in-plane MTJstructure, or an MTJ structure with an arbitrary preferred angle, isused instead.

FIGS. 3A-3D illustrate representative processes for switching the pMTJ200 between the parallel and anti-parallel configurations in accordancewith some implementations. In accordance with some implementations,spin-transfer torque (STT) is used to modify the magnetizationdirections of an MTJ. STT is an effect in which the magnetizationdirection of a ferromagnetic layer in an MTJ is modified by injecting aspin-polarized current into the magnetic element.

In general, electrons possess a spin, a quantized number of angularmomentum intrinsic to the electron. An electrical current is generallyunpolarized, e.g., it consists of 50% spin up and 50% spin downelectrons. When a current is applied though a ferromagnetic layer, theelectrons are polarized with spin orientation corresponding to themagnetization direction of the ferromagnetic layer, thus producing aspin-polarized current (or spin-polarized electrons).

As described earlier, the magnetization direction of the reference layer102 is “fixed” in an MTJ (e.g., the applied currents are insufficient tochange the magnetization state of the reference layer). Therefore,spin-polarized electrons may be used to switch the magnetizationdirection of the storage layer 106 in the MTJ (e.g., switch betweenparallel and anti-parallel configurations).

As will be explained in further detail, when spin-polarized electronstravel to the magnetic region of the storage layer 106 in the MTJ, theelectrons will transfer a portion of their spin-angular momentum to thestorage layer 106, to produce a torque on the magnetization of thestorage layer 106. When sufficient torque is applied, the magnetizationof the storage layer 106 switches, which, in effect, writes either a “1”or a “0” based on whether the storage layer 106 is in the parallel oranti-parallel configuration relative to the reference layer.

FIGS. 3A-3B illustrate the process of switching from the anti-parallelconfiguration to the parallel configuration. In FIG. 3A, the pMTJstructure 200 is in the anti-parallel configuration, e.g., themagnetization direction 302 of the reference layer 102 is opposite tothe magnetization direction 306 of the storage layer 106.

FIG. 3B shows application of a current such that electrons flow throughthe pMTJ 200 in accordance with electron flow 312. The electrons aredirected through the reference layer 102 which has been magnetized withthe magnetization direction 302. As the electrons flow through thereference layer 102, they are polarized (at least in part) by thereference layer 102 and have spin orientation corresponding to themagnetization direction 302 of the reference layer 102. The majority ofthe spin-polarized electrons tunnel through the spacer layer 104 withoutlosing their polarization and subsequently exert torque on theorientation of magnetization of the storage layer 106. When asufficiently large current is applied (e.g., a sufficient number ofpolarized electrons flow into the storage layer 106), the spin torqueflips, or switches, the magnetization direction of the storage layer 106from the magnetization direction 306 in FIG. 3A to the magnetizationdirection 316 in FIG. 3B.

Thus, as shown in FIG. 3B, the magnetization direction 316 of thestorage layer 106 is in the same (upward) direction as the magnetizationdirection 302 of the reference layer 102. Accordingly, the pMTJstructure 200 in FIG. 3B is in the parallel (low resistance state)configuration. In some implementations and instances, electrons thatpossess spins in the minority (opposite) direction are reflected at thebarrier interfaces and exert torque on the magnetization direction 302of the reference layer 102. However, the magnetization direction 302 ofthe reference layer 102 is not switched because the torque, e.g., theamount of electrons, is not sufficient to overcome the damping and henceinsufficient to cause switching in the reference layer 102.

FIGS. 3C-3D illustrate the process of switching from the parallelconfiguration to the anti-parallel configuration. In FIG. 3C, the pMTJstructure 200 is in the parallel configuration. To initiate switching tothe anti-parallel configuration, a current is applied such thatelectrons flow in accordance with electron flow 322 in FIG. 3D. Theelectrons flow from the storage layer 106 to the reference layer 102. Asthe electrons flow through the storage layer 106, they are polarized bythe storage layer 106 and have spin orientation corresponding to themagnetization direction 316 of the storage layer 106.

The MTJ structure 200 in FIG. 3C is in the parallel (low resistancestate) configuration and thus it has lower electrical resistance,therefore, in some implementations and instances, the majority of thespin-polarized electrons tunnel through the spacer layer 104. Minorityspin electrons that are polarized with direction opposite to themagnetization direction 316 of the storage layer 106 are reflected atthe barrier interfaces of the spacer layer 104. The reflected spinelectrons then exert torque on the magnetization 316 of the storagelayer 106, eventually leading to a switch of the magnetization direction316 of the storage layer 106 in FIG. 3C to a magnetization direction 326in FIG. 3D. Thus, the pMTJ structure 200 is switched from the parallel(low resistance state) configuration to the anti-parallel (highresistance state) configuration.

Accordingly, STT allows switching of the magnetization direction of thestorage layer 106. MRAM devices employing STT (e.g., STT-MRAM) offeradvantages including lower power consumption, faster switching, andbetter scalability, over conventional MRAM devices that use magneticfield to switch the magnetization directions. STT-MRAM also offersadvantages over flash memory in that it provides memory cells withlonger life spans (e.g., can be read and written to more times comparedto flash memory).

FIG. 4 is a schematic diagram of a spin transfer torque (STT) MRAMdevice 400 in accordance with some implementations. The includes an MTJdevice with the reference layer 102, the spacer layer 104, the storagelayer 106, and an access transistor 414. The MTJ device is coupled to abit line 408 and a source line 410 via transistor 414, which is operatedby a word line 412. The reference layer 102, the spacer layer 104, andthe storage layer 106 compose the MTJ structure 100 and/or the pMTJstructure 200, as described above with reference to FIGS. 1-3. In someimplementations, the STT-MRAM 400 includes additional read/writecircuitry, one or more additional transistors, one or more senseamplifiers, and/or other components (not shown).

The MTJ structure 100 and/or the pMTJ structure 200 is also sometimesreferred to as an MRAM cell. In some implementations, the STT-MRAM 400contains multiple MRAM cells (e.g., hundreds or thousands of MRAM cells)arranged in an array coupled to respective bit lines and source lines.During a read/write operation, a voltage is applied between the bit line408 and the source line 410 (e.g., corresponding to a “0” or “1” value),and the word line 412 enables current to flow between the bit line 408to the source line 410. In a write operation, the current is sufficientto change a magnetization of the storage layer 106 and thus, dependingon the direction of electron flow, bits of “0” and “1” are written intothe MRAM cell (e.g., as illustrated in FIGS. 3A-3D). In a readoperation, the current is insufficient to change the magnetization ofthe storage layer 106. Instead, a resistance across the MRAM cell isdetermined. e.g., with a low resistance corresponding to a logical “0”and a high resistance corresponding to a logical “1.”

Dual Threshold Voltage Devices

The present disclosure describes various implementations of dualthreshold (e.g., dual gate) voltage devices and systems. As discussed ingreater detail below, dual threshold voltage devices are able to storemultiple bits in a compact layout. Thus, memory arrays can be producedusing the dual threshold voltage devices as memory cells. In addition,dual threshold voltage devices can be implemented as current and/orvoltage selectors for other circuit components, such as magnetic memorydevices. Some magnetic memory devices require inputs with multiplevoltage levels in order to effectively read from and write to thedevices. The dual threshold voltage devices are optionally used (e.g.,in place of larger, more complex analog circuitry) to modulate a voltageor current source so as to provide the required voltage and/or currentlevels. Moreover, the input voltage requirements of some magnetic memorydevices vary with the temperature of the magnetic memory devices. Toachieve a desirable bit error rate (BER) across multiple temperatures,without excessive power consumption, a dual-threshold voltage is used insome implementations to regulate the input voltage/current for themagnetic memory device based on temperature.

FIG. 5 illustrates a device (500) in accordance with someimplementations. In some implementations, the device comprises a core502 and a plurality of layers that surround the core in succession,including a first (e.g., innermost) layer 504, a second layer 506, athird layer 508 and a fourth (e.g., outermost) layer 510. In someimplementations, the core 502, the first layer 504 and the second layer506 constitutes part of a first transistor. In some implementations, thesecond layer 506, the third layer 508 and the fourth layer 510correspond to a second transistor. In some implementations, the secondlayer 506 is a common channel for the first transistor and the secondtransistor. In some implementations, the device further includes a firstinput terminal 512 (e.g., connected to Gate 1) that is coupled to thecore 502 and the first input terminal 512 is configured to receive afirst voltage (e.g., is configured to have a first threshold voltage).In some implementations, the first threshold voltage is based on dopanttype of the channel, dopant level of the channel, dielectric constant offirst dielectric layer, thickness of first dielectric layer, and/or workfunction of the core. In some implementations, the device furtherincludes a second input terminal 514 (e.g., connected to Gate 2) that iscoupled to the fourth layer 510 and the second input terminal 514 isconfigured to receive a second voltage (e.g., is configured to have asecond threshold voltage). In some implementations, the second thresholdvoltage is based on channel dopant type, channel dopant level, thicknessof the third dielectric layer, and/or work function of the fourthconductive layer. The device further comprises a common source terminal518 that is coupled to a source terminal of the channel (e.g., channel804).

In some implementations, the first input terminal 512 and the secondinput terminal 514 are outwardly disposed from the center of the device.In some implementations, there is an angle between the disposition ofthe first input terminal 512 and the second input terminal 514 (e.g.,the first terminal and second terminal are spaced such that they do notshare a vertical plane).

In some implementations, the core is vertical and cylindrical in shape.In some implementations, the plurality of layers annularly surrounds thevertical cylindrical core and the core surrounded by the plurality oflayers creates a cylindrical pillar (e.g., device 500 is shaped as acylindrical pillar). For example, the cylindrical pillar comprises core502, first layer 504, second layer 506, third layer 508, and fourthlayer 510. Further, the cylindrical pillar includes the first inputterminal 512, the second input terminal 514 and the drain terminal 506.In some implementations, the core is composed of electrically lowerresistive material such as tantalum nitride (TaN).

In some implementations, the common source terminal 518 is composed ofsilicided areas coupled to a lower portion of the channel (e.g., thechannel is also coupled to the drain of the device). Commonly, thecylindrical source contact 520 is used and installed to provideelectrical connection to the source 518. Cylindrical source contact 520provides methods to connect source of the dual voltage device to theoutside power supply line.

In some implementations, the core 502 is composed of a conductivematerial, the first layer 504 is a first dielectric layer that surroundsthe core 502, the second layer 506 surrounds the first dielectric layer504, the third layer 508 is a third dielectric layer that surrounds thesecond layer 506 and the fourth layer 510 is composed of a conductivematerial and surrounds the third dielectric layer 508. For example, thefourth layer may be the outermost layer. In some implementations, thefourth layer 510 is composed of a polycide.

FIG. 6 illustrates two implementations of a device (600) in accordancewith some implementations. In some implementations, device 600 hascomponents corresponding to the components of device 500, describedabove, with the first input terminal and the second input terminalaligned to share a vertical plane (e.g., vertically stacked).

In some implementations, the device 600 comprises a core 602 and aplurality of layers that surround the core in succession, including afirst (e.g., innermost) layer 604, a second layer 606, a third layer 608and a fourth (e.g., outermost) layer 610. In some implementations, thecore 602, the first layer 604 and the second layer 606 correspond to afirst transistor. In some implementations, the handle 616 (alsosometimes called contact 616) connected to the 606 drain is not providedand/or is not used (e.g., is not coupled to another device orcomponent). For example, the handle 616 is not used in some MRAM memoryapplications. In some implementations, the second layer 606, the thirdlayer 608 and the fourth layer 610 correspond to a second transistor. Insome implementations, the second layer 606 is a common channel (e.g.,coupled to drain terminal 616) for the first transistor and the secondtransistor. In some implementations, the device further includes aterminal 612, also sometimes called a contact 612, (e.g., Gate 1 inexample (ii)) that is coupled to the core 602 and the terminal 612 isconfigured to receive a first voltage (e.g., is configured to have afirst threshold voltage). In some implementations, the device furtherincludes a terminal 614 (e.g., Gate 2 in example (ii)) that is coupledto the fourth layer 610 and the terminal 614 is configured to receive asecond voltage. The first input terminal 612 and the second inputterminal 614 are stacked on top of each other (e.g., verticallyaligned). In accordance with some implementations, stacking theterminals 612 and 614 enable production of an array of devices with asmaller pitch, as shown in FIG. 24.

In some implementations, an annular device (e.g., device 600) isprovided. The annular device comprises a first transistor including afirst input terminal (e.g., gate 1 handle) and a second transistorincluding a second input terminal (e.g., gate 2 handle). In someimplementations, the first input terminal and the second input terminalextend radially outward from the annular device 600 and the first inputterminal is aligned with the second input terminal.

In some implementations, a magnetic tunnel junction (MTJ) coupled to thechannel drain (e.g., at the top/above the cylindrical core), asdescribed in greater detail with reference to FIG. 8.

In some implementations, the core is vertical and cylindrical in shape,the plurality of layers annularly surrounds the vertical cylindricalcore, and the core surrounded by the plurality of layers creates acylindrical pillar. In some implementations, the first transistor isconfigured to have a first threshold voltage having a first magnitudeand the second transistor is configured to have a second thresholdvoltage having a second magnitude. In some implementations, the secondmagnitude is distinct from the first magnitude.

In some implementations, the first threshold voltage and the secondthreshold voltage are based on a respective thickness for each of thedielectric layer thicknesses and the dopants of the channel and workfunction of the core and the plurality of layers.

In some implementations, the first threshold voltage and the secondthreshold voltage are selected by changing one or more properties of thedevice selected from the group consisting of: a dopant of the device, athickness of one or more of the plurality of layers, and materialcompositions of the first transistor and the second transistor. In someimplementations, the common source terminal is composed of silicidedareas coupled to a bottom plane of the core and the fourth layer.

In some implementations, the core is composed of a conductive material,the first layer is a first dielectric layer that surrounds the core, thesecond layer surrounds the first dielectric layer, the third layer is athird dielectric layer that surrounds the second layer, and the fourthlayer is composed of a conductive material and surrounds the thirddielectric layer.

In some implementations, the fourth layer is composed of a polycide. Insome implementations, the core is composed of a nitride (e.g., TaN). Insome implementations, the device further comprises a cylindrical sourcecontact (e.g., source contact 520) coupled to the common source. In someimplementations, the second layer has a height that is distinct from(e.g., greater than) a height of the core. In some implementations, thesecond layer has a height that is distinct from (e.g., greater than) arespective height of the third layer and the fourth layer.

FIG. 7 shows a schematic circuit that exemplifies operation of thedevice 500. In some implementations, the first transistor (e.g., havinga first gate 706) is configured to have a first threshold voltage (e.g.,Vt1) having a first magnitude and the second transistor (e.g., having asecond gate 708) is configured to have a second threshold voltage (e.g.,Vt2) having a second magnitude. In some implementations, the secondmagnitude is distinct from the first magnitude (e.g., the firstthreshold voltage of the first transistor is distinct from the secondthreshold voltage of the second transistor). For example, the firstthreshold voltage may range from 0.15 V to 2 V and the second thresholdvoltage may range from 0.3V to 3V. In some implementations, thethreshold voltages are negative voltages. In some implementations, gate706 corresponds to the core 502 and gate 708 corresponds to the fourthlayer 510. In some implementations, the drain 702 (e.g., Vd) is coupledto the second layer 506 and a source 704 (e.g., Vs) is coupled to thedevice via the silicide strip 602.

In some implementations, the first voltage and the second voltage arebased on a dopants and their levels of the cylindrical pillar channel.It is to be noted that to have a larger drive current dual Vt device, athicker cylindrical pillar may give a greater drive current. In someimplementations, the first threshold voltage value and the secondthreshold voltage value are selected (e.g., configured) by changing oneor more properties of the device 500. The one or more properties may beselected from the group consisting of a dopant of the device, athickness of one or more of the plurality of layers, and materialcompositions of the first transistor and the second transistor.

FIG. 8 shows an MTJ 802 coupled with the device 500. Device 500 includescore 502 (e.g., gate 706), first layer (not shown), second layer 506(e.g., drain), third layer (not shown) and fourth layer 510 havingcontact terminal 514. Device 500 further includes a silicide source(e.g., common source 518) and a channel 804. Channel 804 may be disposedbetween (e.g., coupled to) silicide source 518 and drain 506 (e.g.,where drain 506 extends through the height of device 500). In someimplementations, the device 500 further comprises a magnetic tunneljunction (MTJ) 802 (e.g., which rests on drain 506).

FIGS. 9-15 illustrate a process for forming a silicide source inaccordance with some implementations. To that end, a method offabricating a device (e.g., an annular device) is provided. The methodcomprises providing a device having two transistors sharing a commonsilicide source. In some implementations, the method comprises providing(e.g., forming) a device 500 having a conductive core corresponding to afirst transistor and a plurality of layers surrounding the core. In someimplementations, the device is a cylindrical device (e.g., cylindricalpillar 600) having a conductive core 502 corresponding to a firsttransistor and a plurality of annular layers surrounding the core,including a first dielectric layer, a second layer, a third dielectriclayer (e.g., the third layer is a dielectric layer), and a fourthconductive layer (e.g., the fourth layer is a conductive layer)corresponding to a second transistor. The method further comprisescreating a silicide source (e.g., silicide source 518), wherein thesilicide source is coupled with the two transistors.

In some implementations, creating the silicide source 518 comprisesdepositing multiple layers in succession on an oxide substrate, themultiple layers including at least an oxide layer and a planarizationlayer. The method further comprises removing, at least partially, theoxide layer and the planarization layer until the oxide substrate isexposed and removing, at least partially, the oxide layer to expose ahorizontal cross section of the cylindrical device to create an annulararea. The method further comprises, after the removing, depositing asiliciding metal on the annular area to form the silicide source.

In some implementations, the method further comprises depositing thesiliciding metal and performing a first rapid thermal annealing (RTA)and then removing the unreacted siliciding metal. In someimplementations, the annular area is at a bottom of the cylindricaldevice.

In some implementations, providing the cylindrical device having the twotransistors comprises: (i) providing a conductive core corresponding toa first transistor of the two transistors, and (ii) forming a pluralityof cylindrical layers around the conductive core, including a firstdielectric layer, a second channel layer, a third dielectric layer, anda fourth conductive layer (e.g., a poly layer) corresponding to a secondtransistor of the two transistors. In some implementations, the secondlayer of the cylindrical device corresponds to a channel. In someimplementations, the method further comprises removing the silicidingmetal from the second layer of the cylindrical device, where removingthe siliciding metal leaves a residual reacted metal forming thesilicide source.

In some implementations, the method further comprises applying a maskprior to removing one or more layers (e.g., before an etching operation)so as to selectively remove portions of the one or more layers. Forexample, a mask could protect the cylindrical core and one or morelayers of the plurality of cylindrical layers during the removing.

In some implementations, forming the plurality of cylindrical layersaround the cylindrical core comprises depositing a spin-on glass (SOG)layer on a first plane of the cylindrical device to create a sloped ringaround the bottom of the cylindrical device, wherein the cylindricaldevice is vertically disposed in the first plane and the SOG layersurrounds the cylindrical device. The method further comprises etchingthe SOG layer from around and inside the cylindrical device to a desireddepth and depositing one or more dielectric materials to form the thirddielectric layer. The method further comprises depositing a dopedmaterial on the first plane, a horizontal cross section of thecylindrical device, a top of the cylindrical device, and the slopedring. The method further comprises etching the doped material on the topof the cylindrical device and on the sloped ring and depositing asiliciding metal to create the fourth conductive layer.

In some implementations, the method further comprises performing a thirdRTA to finish creation of the fourth conductive layer. In someimplementations, the method further comprises wet etching the unreactingsiliciding metal after the third RTA.

In some implementations, the oxide layer is a first oxide layer and themethod further comprises creating a contact to the second transistor ofthe two transistors, including depositing a second oxide layer to createan oxide plane, depositing a spin-on glass (SOG) layer on the oxideplane, wherein the cylindrical device is vertically disposed in theoxide plane, depositing an organic compound to a first height to createan organic planarization layer (OPL), wherein the OPL surrounds ahorizontal cross section of the cylindrical device, depositing a thirdoxide layer onto the OPL, wherein the third oxide layer surrounds ahorizontal cross section of the cylindrical device, depositinganti-reflective coating on the third oxide layer, masking these layersand then removing the deposited layers from areas governed by the masklayout using a first removal technique. Next, the OPL is removed using asecond removal technique until the SOG layer is exposed, thereby forminga trench and depositing a metallic compound in the trench to create thecontact to the second transistor gate.

In some implementations, the method further comprises etching the SOGlayer to a first thickness before depositing the organic compound. Insome implementations, the positioning of the upper and the lower edge ofGate 2 contacts to second transistor are adjusted by varying thethickness of the layers (e.g., varying the thickness of layers 1904,1906, and 1908 in FIGS. 19A and 19B). In some implementations, the firstremoval technique is a fluorine-based chemistry.

In some implementations, the method further comprises depositing aphotoresist layer on the anti-reflective coating, wherein theanti-reflective coating and the photoresist layer surround a horizontalcross section of the cylindrical device.

In some implementations, the method further comprises creating a maskwith the photoresist layer before removing the anti-reflective coatingand the second oxide layer using the fluorine-based chemistry, whereinsaid removing is defined by the mask of the photoresist layer. In someimplementations, the second removal technique uses oxygen plasma toremove the OPL. In some implementations, the compound is tantalumnitride.

In some implementations, the method further comprises creating a contactto the first transistor of the two transistors (e.g., innermost metallicgate), including depositing, on top of the plurality of cylindricallayers, a metal gate contact material, etching through the plurality ofcylindrical layers and the metal gate contact material to a first heightextending across the conductive core through the fourth layer (i.e.,forming a trench), depositing a first mask, depositing a layer ofsilicon nitride (SiN) (e.g., in the formed trench), depositing a secondmask and etching the layer of SiN in accordance with the second mask tocreate a flat surface, and depositing a metal gate contact on the flatsurface to create a contact to the first transistor. Using the SiNensures that the metal gate contact (e.g., Gate 1 contact metal) doesnot short any conductive annular layers of either of the transistors.

In some implementations, the metal gate contact material is tantalumnitride. In some implementations, the first height is based on theheight of a second highest layer of the plurality of layers. In someimplementations, the method further comprises, after depositing thefirst mask, wet dipping the plurality of layers in potassium hydroxidein accordance with the mask. In some implementations, the wet dippingdecreases the height of the third layer and the fourth layer. In someimplementations, the layer of silicon nitride is thick SiN. In someimplementations, the method further comprises performing a chemicalmechanical polishing (CMP) on the layer of SiN.

In some implementations, the process starts with Silicon (Si) (e.g.,Silicon having a (110) lattice structure), and the process comprisesdepositing or growing and converting a small part of Silicon itself toits Silicon dioxide compound, a thin oxide layer onto the Si to create aplanar layer 902, as shown in FIG. 9A, which provides necessary maskingmaterials to etch shallow trench isolation and active transistor/deviceareas. The process further comprises creating a mask with a shallowtrench isolation (STI) 908 and a channel 904 (e.g., a cylindricalchannel), and removing the Si (e.g., by dry etching and/or wet etchingthe Si), as shown in FIG. 9B. In some implementations, the channel 904corresponds to channel 804 as shown in FIG. 8. As shown in FIG. 9C, theprocess further comprises growing the oxide to the required thicknessfor silicide formation (e.g., to create oxide-covered cylindricalchannel 906). For example, the oxide will grow at the bottom of thecylindrical channel, on the inside of the cylinder, and on the outsideof the cylinder.

FIG. 10A illustrates doping a P-device (e.g., PMOS) and/or an N-device(e.g., NMOS) to configure the transistors to proper magnitude (e.g., topositive or negative threshold voltages) (e.g., the first transistor isconfigured to have a first threshold voltage and the second transistoris configured to have a second threshold voltage). For example, theprocess may further comprise creating a mask and doping the P-device(e.g., P pillar) with Boron high angled multiple Ion Implant in order toadjust the threshold voltage of the P-device. In some implementations,the process may further comprise creating a mask and doping the N-device(e.g., N pillar) with Phosphorous high angled multiple Ion Implant inorder to adjust the threshold voltage of the N-device. In someimplementations, the method further comprises performing rapid thermalannealing (RTA) to activate the dopants in the P-device and/or N-device.In some implementations, the outside and inside of the pillars (e.g.,barrels) are doped in order to adjust the threshold voltage of thepillars. In some implementations, the mobility of the semiconductormaterial affects the threshold voltage. It is noted that other dopingagents could also be used for the doping operation explained above, andthe examples provided are merely one set of possible doping agents. Insome implementations, the process creates a P-device or an N-device. Insome implementations, both a P-device and an N-device may be created, asdescribed with reference to FIG. 14.

FIG. 11 illustrates the process of silicide formation of the commonsource. FIGS. 11A and 11B show the annular channel 906 (e.g., the commonchannel extending from below the so called source to the top layercalled the drain). Creating the silicide source includes depositing aplurality of layers in succession on an oxide substrate (e.g., the oxidegrown as described with respect to FIG. 9). The plurality of layersincludes at least an oxide layer and a polymerizing layer (e.g., anorganic polymerizing layer (OPL) 1102). In some implementations, asdiscussed below, the layers may also include a low temperature siliconoxide (LTO) layer, an antireflective layer, and/or a photoresistivelayer. The process further includes removing, at least partially, theoxide layer and the polymerizing layer until the oxide substrate isexposed using a first removal technique (e.g., dry etching). In someimplementations, the removal is in accordance with a mask (e.g., mask1204, FIG. 12B) over the second layer. The process further includesremoving, at least partially, the oxide layer to expose a layer ofsilicon on a horizontal cross section of the device using a secondremoval technique (e.g., which may be the same technique as the firstremoval technique or may be a different technique, such as dry and/orwet etching). In some implementations, where the device is a cylindricaldevice this creates an annular area 1302 (e.g., at the bottom of thedevice). After the Ion Implant for the source and the removing of thePhotoresist, the process includes performing a first rapid thermalannealing (RTA) on the area which creates a low resistivity contact tothe source of either transistor. Then, depositing a siliciding metal onthe area to form the silicide source. The process further includesperforming a second RTA, which produces a very low resistive contacts tosource areas of both transistors.

In some implementations, the process of formation of the sourcesilicidation includes coating (e.g., depositing) a first oxide (e.g., athin film) on silicon, then coating it with an organic polymerizinglayer (OPL) to create an OPL plane 1102, wherein the OPL plane 1102surrounds a horizontal cross section of the cylindrical device 906. Insome implementations, the polymerizing layer (e.g., OPL) is thinned downto a required thickness for the silicide formation. The process furtherincludes depositing low-temperature silicon oxide (LTO) on the OPL tocreate an LTO layer 1104, wherein the LTO layer 1104 surrounds ahorizontal cross section of the cylindrical device 906. The processfurther includes coating the LTO layer 1104 with a bottom antireflectivecoating (BARC) layer and a photoresistive (PR) layer 1106, as shown inFIG. 11C. The process further includes applying a layer of silicide1202.

In some implementations, the process further includes creating a mask1204 with the PR layer, wherein the mask 1204 covers the second layer(e.g., channel) of the cylindrical device 906. The process furtherincludes dry etching (e.g., using a high pressure controlled dry etch)the BARC layer, the LTO layer and the OPL until the oxide substrate 1206is exposed. For example, the center of the cylindrical channel iscovered with masking PR such that the center of the channel barrel isnot exposed (e.g., is not effected) for this dry etching or thefollowing etching and siliciding. In some implementations, the dryetching is fluorine-based (e.g, NF3 and/or SF6). The process furtherincludes, as shown in FIG. 13, etching horizontally the LTO layer andoxide (e.g., at the bottom of the device) to expose a layer of siliconon a horizontal cross section of the cylindrical device to create anannular area 1302. For example, wet etching may be used for etching theLTO layer on the outside of the cylinder. The process further includesIon Implanting the source dopant and performing a first rapid thermalanneal (RTA) processing on the annular area 1302 (e.g., the insideand/or outside of the channel above the annular area are not exposed tosource ion implant and are covered in PR). In some implementations, theannular channel is unaffected by the Ion Implant processing, and themetal siliciding process. In some implementations, the PR layer and OPLis stripped off (e.g., the inside of the channel is oxide).

In some implementations, the horizontal etching is wet etching and/ordry etching (e.g., performed sequentially). In some implementations, thesecond layer of the cylindrical device corresponds to a channel and thechannel is not exposed to the etching of the BARC layer, the LTO layerand the OPL.

The process further includes depositing a siliciding metal (e.g., Ti 50nm) on the annular area 1302 to create the silicide source andperforming a second Rapid Thermal Annealing. In some implementations,the annular area 1302 is at the bottom of the cylindrical device.

FIG. 14 a schematic view of both an N-device and a P-device. The viewincludes STI 908-1 and 908-2, a silicon P-device and a silicon N-deviceput together, for example, to create a CMOS device. In someimplementations, an N-device and a P-device may both be configured andfabricated and in other configurations, either type of devices can bemade. FIG. 14 also illustrates the device core 502 and source terminal518 that is coupled to the device core 502. STI areas 908-1 and 908-2serve to isolate the neighboring devices from each other.

FIGS. 15A, 15B and 15C show siliciding process steps in accordance withsome implementations. FIG. 15A shows an oxide layer covering most of thearea but a small portion of area shows exposed Silicon. FIG. 15B showsthe surface areas after depositing the siliciding metal all over. Here,only a small area of Silicon is in direct contact with siliciding metalwhich converts to low resistivity silicide area. FIG. 15C, the area 1504and the top annular Silicon area become low resistivity material ofmetal silicide. In some implementations, the siliciding metal that isdeposited is titanium or another metal. The siliciding metal connects(e.g., mixes and/or reacts) with areas that have silicon exposed (e.g.,including the annular area 1302, FIG. 13A). For example, the silicidingmetal may be deposited over the entire device, but the siliciding metalmay only fuse the previously exposed silicon along strip 1504 (e.g., anannular area 1302, FIG. 13A) and the top annular area of FIG. 15C.

In some implementations, the method further comprises removing thesiliciding metal from the second layer 506 (e.g., the channel drain) ofthe cylindrical device (e.g., using hydrogen peroxide or anothercompatible chemical), wherein the removal leaves a residual reactedmetal along strip 1504, forming (e.g., leaving) the silicide source. Insome implementations, removing the unreacted siliciding metal (e.g.,unreacted Ti) includes wet etching the metal.

FIGS. 16-18 illustrate process states for forming a Gate 2 structure forthe transistor in accordance with some implementations. For example, amethod is provided to form a wrap-around annular poly gate (e.g., a gateof a transistor), such as, in some implementations, the fourth layer 510shown in FIG. 5. In some implementations, the method comprises forming adevice 500 (e.g., a cylindrical device) having a conductive core 502corresponding to a first transistor (e.g., a first gate of a firsttransistor) and a plurality of annular layers surrounding the core 502,including a first dielectric layer 504, a second layer 506, a thirddielectric layer 508, and a fourth conductive layer 510 corresponding toa second transistor (e.g., a second gate of a second transistor). Forexample, the cylindrical channel shown in FIG. 16A is provided and thefollowing process describes forming the fourth layer around thecylindrical channel. In some implementations, any remaining SOG fromprevious processing is stripped off in FIG. 16A.

The method comprises coating (e.g., depositing) a spin-on glass (SOG)layer 1604 on a first plane (e.g., on which the device rests) andcoating the device to create a sloped ring 1606 (e.g., or other slopedshape surrounding the shape of the device 500, such as a square) aroundthe bottom of the cylindrical device, wherein the device (e.g.,cylindrical device) is vertically disposed in the first plane and theSOG surrounds the cylindrical device. The method further comprisesetching (e.g., removing) the SOG layer from around and inside thecylindrical device to a desired depth and depositing dielectricmaterials to form the third dielectric layer 1608 (e.g., the third layeris a dielectric layer) (e.g., such that oxide grows on all surfaces ofthe cylindrical device) to create the device shown in FIG. 16C. Themethod further includes depositing doped material (e.g., a chemicallyvaporized deposit (CVD) polysilicon 1702) (e.g., the dark shading) onthe first plane, a horizontal cross section of the device, a top of thedevice 1704, and the sloped ring 1606 around the device, shown in FIG.17B. The method further comprises etching (e.g., using a reactive-ionetch (RIE)), the doped material (e.g., polysilicon) on the top of thedevice and on the sloped ring 1606, shown in FIG. 18 (e.g., thepolysilicon on the sloped ring and the top of the cylinder has beenremoved). The method further comprises depositing a siliciding metal tocreate the fourth conductive layer of low resistivity. In someimplementations, the method further comprises performing rapid thermalanneal (RTA) to create the fourth conductive layer of low resistivity.In some implementations, the inside of the channel is also silicided(not shown).

In some implementations, the method further includes etching (e.g., wetetching) the unreacted siliciding metal after performing the RTA. Insome implementations, the device is a cylindrical device and theplurality of layers are annular layers.

FIGS. 19A-19B illustrates a process for creating a gate handle for atransistor in accordance with some implementations. For example, amethod is provided for creating a first (e.g., or second) transistorgate handle (e.g. second input terminal 514) of the two transistors. Insome implementations, the method includes coupling a channel (e.g., thechannel is also coupled to the drain/second layer of the device) of thecylindrical device to a silicide source (e.g., coupled at the bottom ofthe cylindrical device). For example, the channel is disposed betweenthe drain and the source. In some implementations, the method offabricating an annular device includes providing a device having twotransistors. For example, providing a cylindrical device having aconductive core 502 corresponding to a first transistor and a pluralityof annular layers surrounding the core 502, including a first dielectriclayer 504, a second layer 506, a third dielectric layer 508, and afourth conductive layer 510 corresponding to a second transistor,wherein the fourth conductive layer is an outermost layer of thecylindrical device.

The method further includes creating a gate handle, including depositinga first oxide layer to create a first oxide plane 1902. In someimplementations, the first oxide layer is created by depositing a firstlow-temperature oxide (LTO) layer. The method further includesdepositing a spin-on glass (SOG) layer 1904 on the first oxide plane1902, wherein the device 500 is vertically disposed in the first oxideplane 1902. In some implementations, the method further includes etching(e.g., reactive-ion etching) the SOG layer 1904 to a first thickness.

The method further includes spin coating (e.g., depositing) an organiccompound to a first height to create an organic planarizing layer (OPL)1906, wherein the OPL 1906 surrounds a horizontal cross section of thecylindrical device 500. The method further includes depositing a secondoxide (e.g., LTO) layer 1908 onto the OPL 1906 and depositinganti-reflective coating on the second oxide layer. In someimplementations the second oxide (e.g., LTO) layer 1908 surrounds ahorizontal cross section (e.g., above the horizontal cross sectionsurrounded by the OPL 1906) of the cylindrical device 500. In someimplementations, the method includes depositing (e.g., coating) thesecond LTO layer 1908 with a bottom anti-reflective coating (BARC) layer1910 and coating the BARC layer 1910 with a photoresist layer 1912,wherein the BARC layer 1910 and the photoresist layer 1912 surround ahorizontal cross section of the cylindrical device (e.g., above thehorizontal cross sections of the device surrounded by the OPL layers andLTO layer).

FIG. 19A shows the device before masking and etching the OPL withvarying thickness of the layers 1912, 1908, and 1906. The variedthickness of the layers enables contacts of varying dimensions to becoupled to the outer layer (e.g., the annular wrap-around poly silicide)of the device 500. For example, the thicknesses in example (i) enable acontact that spans most of the length of the device 500 (e.g., thecontact 514 in FIG. 5). As another example, the thicknesses in example(iii) enable a contact configured to stack with other contacts in thevertical plane (e.g., the Gate 2 contact in FIG. 6). FIG. 19B shows thedevice after masking and etching the OPL (e.g., such that a long trenchis formed for a gate handle for the second transistor to be filled in).The example (i) in FIG. 19B corresponds to example (i) in FIG. 19A andthe example (ii) in FIG. 19B corresponds to the example (iii) in FIG.19A.

The method includes removing, using a first removal technique theanti-reflective coating and the second oxide layer. For example, themethod may further include creating a mask with the photoresist layerand dry etching the BARC layer and the second LTO layer using afluorine-based chemistry (e.g., the first removal technique) through theBARC layer and the second LTO layer into the OPL (e.g., the top portionof the OPL without etching through the OPL completely), wherein the dryetching is defined by the mask of the photoresistive layer. The methodfurther includes removing, using a second removal technique (e.g., whichmay be the same or different than the first removal technique), the OPL(e.g., etching in oxygen plasma) until the SOG layer 1904 is exposed tocreate a trench 1914. The method further includes depositing tantalumnitride (e.g., or other conductive material) in the trench 1914 tocreate a gate (e.g., second input terminal 514) that extends outwardlyfrom the fourth conductive layer.

In some implementations, the photoresistive layer and the BARC layersare completely etched off. In some implementations, the silicide sourceis coupled to the channel (e.g., which is also coupled to thedrain/second layer) and is not coupled to the remaining plurality ofannular layers (e.g., the silicide source is only coupled to the channeldrain without being coupled to the remaining plurality of layers). Forexample, the silicide source is selectively coupled to prevent ashorting (e.g., of the circuit) of the device.

FIGS. 20A-20D illustrates a schematic of the device having a contact(e.g., a gate handle contact) to an annular transistor. FIG. 20Aillustrates the device after repeatedly depositing and etching to fillin the second input terminal (e.g., by filling trench 1914 with TaN) andthe source contact 520. Then, a CMP of the TaN may be performed. Next,the process may include etching LTO layer and performing an oxygenplasma ash off the OPL. FIG. 20B illustrates the device after thecontacts have been etched. In some implementations, two masks or asingle combined mask may be used to etch the contacts. FIG. 20Cillustrates depositing and etching repeatedly to fill in the handle tothe fourth layer (e.g., contacting the fourth layer with the secondinput terminal 514) and to fully fill in the source contact 520. FIG.20D illustrates various schematics of the device with and without SOGand LTO layers. The TaN may be recessed, as shown as the bottom image ofFIG. 20D, by selective wet or dry etching followed by filling therecessed areas with silicon oxide (SiO2) and CMP.

FIGS. 20A-20D also show a contact to the conductive core (e.g., inaddition to the gate handle contact to the annular transistor describedabove). A method for creating a contact to the first transistor (e.g.,the conductive core 502) may include depositing (e.g., by chemical vapordeposition (CVD)), a first silicon nitride (SiN) layer to create a firstSiN plane. The method for further comprises depositing a spin-on glass(SOG) layer and an oxide (e.g., a low temperature oxide (LTO)) layer onthe first SiN plane. In some implementations, the method furtherincludes performing a metal gate contact mask to define the contact tothe first transistor. The method further includes etching (e.g., usingan ion beam trench etch), to create a trench at a required depth throughthe plurality of layers. The method further includes depositing a secondSiN layer in the trench 2102 and on sides of the trench and removing, atleast partially, the deposited first SiN layer from the core. In someimplementations, the removal is performed by a first chemical mechanicalpolishing (CMP). The method further includes depositing a metal gatecontact to create the contact to the first transistor (e.g., conductivecore). For example, this method makes the bottom of the trench aninsulating layer of Silicon Nitride, which prevents the shorting ofunderneath various metal layers. In some implementations, the metal gatecontact is composed of Tantalum Nitride (TaN). In some implementations,the metal gate contact material (e.g., TaN) is deposited over theplurality of layers.

FIGS. 20E-20G illustrate a device with given heights and planarizing thedevice. As shown in FIG. 20E, the core (e.g., TaN G1) is lower than thechannel drain (e.g., the second layer). In some implementations, thechannel drain has the greatest height. FIG. 20F depicts how to planarizevarious electrodes by depositing TaN Gate 1 electrode over the surfaceof the device and plane, followed by performing a CMP of the TaN, asshown in FIG. 20G.

FIGS. 20H-20K illustrate a process for adjusting heights of a device. Insome implementations, the drain (e.g., second layer 506) is higher inthe device than the core 502 and the plurality of other layers,including layer 504, 508 and 510. SiN 2020 is deposited over the layersand core. Then, a CMP is performed on the SiN to expose the drain 506,such that a metal contact may be deposited onto the drain, as shown inthe top two figures of FIG. 20H. Thus, a contact to the drain when thedrain is the highest layer is possible. In order to contact the core 502(e.g., which is lower in height than the drain), a layer of SiN 2022 isdeposited over the layers and core. Then, a mask is applied to open thecontact to the core 502 (e.g., the gate of the first transistor) and ametal damascene is applied.

FIGS. 20I-20K illustrate a process of contacting the core when the core502 is the tallest height of the device. A layer of SiN 2024 isdeposited. Then, a CMP is prfomed on the SiN 2024 to reduce its heighton core 502, as shown in FIG. 20J. Next, masking and etching a trenchhandle is performed. The trench is then filled in with metal and a CMPis performed to create first input terminal 512.

FIGS. 21-23 illustrate a process for contacting (e.g., creating a firstinput terminal) a transistor located in the center of a device inaccordance with some implementations. FIG. 21A is a cross-sectional viewof the plurality of layers surrounding the core 502. In someimplementations, the core 502 (e.g., the first transistor gate) is thelowest in height of the layers of the device. FIGS. 21B-21G illustrateanother cross-sectional view of the device. As shown in FIG. 21C, alayer of metal gate material (e.g., TaN 2106) is deposited. Then, a CMPof the TaN is performed to the second highest layer (e.g., the fourthlayer 510 is the second highest in this example), as shown in FIG. 21D.Next, mask with a first mask handle for the first gate (e.g., mask forthe first input terminal 512), perform a wet dip in potassium hydroxideand Tetra Methyal Ammonium Hydroxide (TMAH) SOL to lower the heights ofsilicon and polysilicon in the first handle area (e.g., where the firstinput terminal 512 will be) only. TMAH is an etchant that etches Siliconand polysilicon without etching the oxide layer(s). Then, deposit thickSiN 2108, as shown in FIG. 21F. Perform a CMP on the SiN 2108 to leavethe SiN 2108 to a 200A thickness on a TaN pad over core 502, as shown inFIG. 21G.

In some implementations, a method of forming a contact (e.g., firstinput terminal and/or second input terminal and/or drain terminal)comprises, at a device (e.g., an annular device) composed of a core anda plurality of layers that surround the core in succession, theplurality of layers including a first layer and a second layer, whereinthe core and the second layer are separated by the first layer,depositing a coating on a surface of the annular device, the surfaceincluding the core and the plurality of layers. The method furthercomprises determining relative heights of the core, the first layer, andthe second layer. In accordance with a determination that the secondlayer has the largest relative height, the method comprises performing afirst removal of the coating to expose the second layer, depositing afirst metal on the second layer, performing a second removal of thecoating to expose the core, and depositing a second metal on the core.In accordance with a determination that the core has the largestrelative height, the method comprises etching a portion of the coatingto expose the core and forming a first terminal by depositing aconductive material, wherein the conductive material contacts theexposed core and the first input terminal extends radially outward fromthe annular device.

In some implementations, the method further comprises, after forming thefirst input terminal, planarizing a top surface of the first inputterminal to create a flat surface. In some implementations, the firstremoval is a planarization (e.g., polishing) process. In someimplementations, the first removal is a chemical mechanicalplanarization process. In some implementations, the coating is anitride-based coating. In some implementations, the plurality of layersfurther includes a third layer and a fourth layer. In someimplementations, the core, the first layer, and the second layercorrespond to a first transistor, the second layer, the third layer, andthe fourth layer correspond to a second transistor, and the second layeris a channel drain common to the first transistor and the secondtransistor. In some implementations, the first metal is the secondmetal.

FIG. 22 illustrates three-dimensional views of creating the contact withcore 502. FIG. 22A is a three-dimensional view of the device after theCMP of SiN 2108 before performing a second mask handle for the firstgate (e.g., first input terminal 512). FIG. 22B is a three-dimensionalview after performing the second masking and etching of SiN down to themetal of the first gate of the first transistor, as shown by the opentrench 2102. FIG. 22C illustrates a three-dimensional view of the handleafter depositing and performing a CMP of metal 2110 to create the firstinput terminal 512 coupled with core 502.

In some implementations, the method of creating a contact with the core,as shown in FIG. 22, comprises providing a device having twotransistors, where the device comprises (i) a conductive corecorresponding to a first transistor of the two transistors and (ii) aplurality of layers surrounding the core. In some implementations, acylindrical device having a conductive core 502 corresponding to a firsttransistor and a plurality of annular layers surrounding the core,including a first dielectric layer 504, a second layer 506, a thirddielectric layer 508, and a fourth conductive layer 510 corresponding toa second transistor is provided. In some implementations, the core 502(e.g., Gate 1) is the lowest in height (e.g., shortest), as shown inFIG. 23A.

In some implementations, handle 2110 has various thicknesses as itcrosses the cross-sections of the plurality of layers. For example, thehandle 2110 shown in FIG. 22C may be narrower at the core 502 and mayincrease in thickness outwardly (e.g., through the plurality of layers).

FIG. 24 is a diagram of an array 2400 of dual threshold voltage devicesin accordance with some implementations. The array 2400 includes aplurality of dual threshold voltage devices shown in example (i) (e.g.,the device 600, FIG. 6). In the array 2400 the contact handle to thegate 1 of the first transistor and the contact handle to gate 2 of thesecond transistor are vertically aligned. A vertically-alignedconfiguration is desirable as it enables a higher density layout asshown in FIG. 24 (e.g., for higher density MRAM and other memory deviceslayout). In some implementations, the layout size is determined by aminimum size of the core (e.g., the core 502) and minimum size of thedrain. In some implementations, the unit cell size of a device isgreater than 25F{circumflex over ( )}2, where F represents a unit size(e.g., based on the fabrication process). The wordline 2406 couples tothe Gate 1 contact and the wordline 2408 couples to the Gate 2 contactin accordance with some implementations. The wordline 2406 couples tothe Gate 1 contact of each device in the first row and the wordline 2408couples to the Gate 2 contact of each device in the first row inaccordance with some implementations. As shown in FIG. 24(i), thewordline 2406 and the wordline 2408 optionally overlap by stacking gate1 and gate 2 vertically (e.g., without bridging them together) inaccordance with some implementations. The source line 2402 couples to asource contact (e.g., the source contact 520, FIG. 5, or the sourcecontact 618, FIG. 6) of each device in a column of devices, and thedrain line 2402 couples to a drain contact (e.g., the drain contact 616,FIG. 6) of each device in the column of devices. Thus, in someimplementations, the devices are arranged with a 4F pitch as shown inFIG. 24.

Although some of various drawings illustrate a number of logical stagesin a particular order, stages that are not order dependent may bereordered and other stages may be combined or broken out. While somereordering or other groupings are specifically mentioned, others will beobvious to those of ordinary skill in the art, so the ordering andgroupings presented herein are not an exhaustive list of alternatives.Moreover, it should be recognized that the stages could be implementedin hardware, firmware, software or any combination thereof.

It will also be understood that, although the terms first, second, etc.are, in some instances, used herein to describe various elements, theseelements should not be limited by these terms. These terms are only usedto distinguish one element from another. For example, a first devicecould be termed a second device, and, similarly, a second device couldbe termed a first device, without departing from the scope of thevarious described implementations. The first device and the seconddevice are both electronic devices, but they are not the same deviceunless it is explicitly stated otherwise.

The terminology used in the description of the various describedimplementations herein is for the purpose of describing particularimplementations only and is not intended to be limiting. As used in thedescription of the various described implementations and the appendedclaims, the singular forms “a”, “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will also be understood that the term “and/or” as usedherein refers to and encompasses any and all possible combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “includes,” “including,” “comprises,” and/or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

As used herein, the term “if” is, optionally, construed to mean “when”or “upon” or “in response to determining” or “in response to detecting”or “in accordance with a determination that,” depending on the context.Similarly, the phrase “if it is determined” or “if [a stated conditionor event] is detected” is, optionally, construed to mean “upondetermining” or “in response to determining” or “upon detecting [thestated condition or event]” or “in response to detecting [the statedcondition or event]” or “in accordance with a determination that [astated condition or event] is detected,” depending on the context.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific implementations. However, theillustrative discussions above are not intended to be exhaustive or tolimit the scope of the claims to the precise forms disclosed. Manymodifications and variations are possible in view of the aboveteachings. The implementations were chosen in order to best explain theprinciples underlying the claims and their practical applications, tothereby enable others skilled in the art to best use the implementationswith various modifications as are suited to the particular usescontemplated.

What is claimed is:
 1. An annular device, comprising: a cylindricalpillar, comprising: a vertical cylindrical core; and a plurality oflayers that surround the vertical cylindrical core in succession, theplurality of layers including a first layer, a second layer, a thirdlayer, and a fourth layer, wherein: the core, the first layer, and thesecond layer correspond to a first transistor including a first inputterminal, wherein the first transistor is configured to have a firstthreshold voltage having a first magnitude; and the second layer, thethird layer, and the fourth layer correspond to a second transistorincluding a second input terminal, wherein the second transistor isconfigured to have a second threshold voltage having a second magnitudedistinct from the first magnitude.
 2. The annular device of claim 1,wherein: the second layer is a common channel having a channel sourceand a channel drain; the first input terminal is coupled to the core,the first input terminal being configured to receive a first voltage forthe first transistor; and the second input terminal is coupled to thefourth layer, the second input terminal being configured to receive asecond voltage for the second transistor.
 3. The annular device of claim2, further comprising a magnetic tunnel junction (MTJ) coupled to thechannel drain.
 4. The annular device of claim 2, further comprising acylindrical source contact coupled to the channel source.
 5. The annulardevice of claim 3, wherein: the MTJ includes a storage layer, a spacerlayer, and a reference layer; and the reference layer of the MTJ iscoupled to the channel drain.
 6. The annular device of claim 1, wherein:the first layer is a first dielectric layer; the third layer is a thirddielectric layer; and the first threshold voltage and the secondthreshold voltage are based on respective thicknesses for the firstdielectric layer and the third dielectric layer.
 7. The annular deviceof claim 1, wherein the first threshold voltage and the second thresholdvoltage are selected by changing one or more properties of the annulardevice selected from the group consisting of: a dopant of the annulardevice, a material composition of the first input terminal, a workfunction of the first input terminal, a material composition of thesecond input terminal, and a work function of the second input terminal.8. The annular device of claim 1, wherein: the core is conductive; thefirst layer is a first dielectric layer that surrounds the core; thesecond layer surrounds the first dielectric layer and is composed of asemiconductor material; the third layer is a third dielectric layer thatsurrounds the second layer; and the fourth layer is conductive andsurrounds the third dielectric layer.
 9. The annular device of claim 1,wherein the fourth layer is composed of a polycide.
 10. The annulardevice of claim 1, wherein the core is composed of a Tantalum material.11. The annular device of claim 1, wherein the second layer has a heightthat is distinct from a height of the core.
 12. The annular device ofclaim 1, wherein the second layer has a height that is distinct from arespective height of the third layer and the fourth layer.
 13. Theannular device of claim 1, wherein the first input terminal and thesecond input terminal extend radially outward from the annular device.14. The annular device of claim 1, wherein the first input terminal isaligned with the second input terminal.
 15. An annular device,comprising: a core; and a plurality of layers that surround the core insuccession, the plurality of layers including a first layer, a secondlayer, a third layer, and a fourth layer, wherein: the core, the firstlayer, and the second layer correspond to a first transistor includinghaving a first input terminal; and the second layer, the third layer,and the fourth layer correspond to a second transistor including havinga second input terminal; and the second layer is a common channel havinga channel source and a channel drain; the first input terminal iscoupled to the core, the first input terminal being configured toreceive a first voltage for the first transistor; and the second inputterminal is coupled to the fourth layer, the second input terminal beingconfigured to receive a second voltage for the second transistor.